If you do not have the roll bar already, I would get it first. First of all it is a safety item. You want to keep your pumpkin as along a possible! Also, it adds a lot of rigidity to the rear as a bonus. After I put mine on, I could feel the suspension was actually working rather than the flex of the chassis. Next thing to help with the chassis flex is to upgrade your suspension to keep if off the bump stops. Miata, at least my NB, rides very close the rear bump stops. Mine 99's were totally gone after 145K miles! After I done the GC/Koni combo, I no longer hear the chassis flexing noises when backing into the sloping alley. I also cure the axle hop on wet surface.